Practical Wiring - Club Level: Grounding Design
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Grounding Design
08.30
| 00:00 | - By a large margin most automotive electrical gremlins are caused by poor grounding. |
| 00:06 | We need to be careful with how we ground the components of our EFI system as an incorrect grounding scheme can have unpredictable consequences. |
| 00:13 | Particularly once the car has been on the road or track for a couple of years or is modified in the future and a connection corrodes, is missed, or forgotten. |
| 00:20 | As detailed in the wiring fundamentals course, the primary consideration we need to keep in our minds is eliminating common ground paths. |
| 00:27 | Or a ground current having more than one way of getting back to the battery or engine block. |
| 00:32 | If we limit our ground currents to only one path, we can be sure of where they will run and if a connection corrodes or vibrates loose, the EFI system component will fail in a predictable way, rather than an electrical gremlin making itself known. |
| 00:46 | We achieve this by a technique called star point earthing. |
| 00:50 | Much like with our power supply design process, the first step of our grounding design is to list all of the components that will need a ground connection. |
| 00:57 | This will usually be fewer components than we have in our power supply list as many EFI system actuators are actually low side switched. |
| 01:05 | Meaning they're given power supply from our power supply scheme, but are given a ground connection by another device which allows current to flow through them, turning them on. |
| 01:13 | Fuel injectors are the most common examples of this. |
| 01:16 | We supply power to them from our enable relay, but they're given a ground connection by the ECU when they're required to open. |
| 01:22 | And they do not get directly connected to ground themselves. |
| 01:26 | This leads us to another point in that some components in the EFI system will need either larger gauge wires or more wires of the same gauge for their ground connections as compared to their power supply connections. |
| 01:38 | The ECU is the best example of this as it usually only needs a fairly small power supply connection for its internal circuitry. |
| 01:45 | But as it passes the current from the fuel injectors and other system actuators to ground, it requires larger ground connections. |
| 01:52 | This is why you often find aftermarket ECUs are designed with more power ground pins that power supply pins. |
| 01:58 | The most common grounding scheme and the one we are going to implement for our FD3S example harness has two star earthing points. |
| 02:05 | Being the engine block and the vehicle chassis. |
| 02:07 | As we're using two star earth points, we need to be careful about how we connect these points together and ensure that there is only one connection between them. |
| 02:15 | This entails ensuring that any electrical component which requires a ground is connected to either the engine block or the chassis but never both at the same time. |
| 02:25 | We then make the connection between then engine block and chassis using a large power cable called an earth strap that's usually constructed with two AWG or 35 millimetre square power cable and tinned solid copper crimp lugs on either end. |
| 02:39 | Another of these power cables will then make a connection from the engine block to the battery negative terminal. |
| 02:44 | By following this scheme we can be certain that any power ground current will flow where we intend it to and not cause us any unexpected issues. |
| 02:52 | Often it is possible to reuse the OEM ground strap and battery negative cable configuration however in our situation, with the FD3S RX-7, these are a known weak point and we will construct our own cables from scratch. |
| 03:06 | As these earth cables are large and often a little bulky, they do not get integrated into the rest of our wiring harness, instead being left as a standalone item. |
| 03:14 | Now that our star earth points are defined, as well as the connection between them, we need to give a ground connection to all of the other parts of the EFI system which require one. |
| 03:23 | This is where we need to analyse all of the connections and make sure that a device is grounded to only one star earthing point. |
| 03:29 | On the surface, this seems relatively easy as we can simply run the ground wires from our device to either the engine block or the chassis, whichever is easier. |
| 03:38 | However this does not always work as sometimes the component in our EFI system might make an unexpected ground connection through its body or mounting location. |
| 03:46 | A good example of this is the Nissan engine speed sensors, as used on the RB and SR series of engines. |
| 03:53 | These units require a power supply and a power ground for their internal circuitry. |
| 03:57 | But their power ground pin is connected internally to their aluminium body, which is then bolted to the engine block. |
| 04:04 | This means that if we run our power ground wire from the sensor connector, back to a grounding point on the chassis, it's now grounded to both the engine block and the chassis which is what we are trying to avoid. |
| 04:15 | For this reason, the engine block is my preferred grounding location, as it eliminates the majority of these situations with the chassis only being used when running a power ground wire to the engine block being practical. |
| 04:26 | Such as in the case of a fuel pump where it's mounted at the other end of the vehicle. |
| 04:31 | Once we have determined where we are going to ground our EFI components to, we need to size the ground wires, in much the same way as we did for our power supply scheme. |
| 04:40 | As current flows in a continuous loop, the amount of current entering your device is usually the same the amount of current leaving the device. |
| 04:47 | And we use this rule to size our wiring. |
| 04:49 | Typically this results in us using the same size wire for both the power supply and power ground. |
| 04:55 | Where we need to be careful however is where one EFI component is providing a ground connection for another. |
| 05:01 | Looking at our fuel injector example again, we know that there will be the same amount of current entering the injector as leaving it. |
| 05:07 | Meaning we will use the same size wire to connect the injector to our ECU as we have to supply with power from our enable relay. |
| 05:14 | However our ECU will now be passing the injector current to ground so it will have more current flowing out of it than is flowing into it through its own power supply pins. |
| 05:25 | This will mean it will need either more power ground wires than supply wires, or a larger gauge of wire used. |
| 05:31 | This is often catered for by the ECU having more than one power ground pin. |
| 05:35 | It's important that these power ground pins are run back to our grounding point individually or if they are spliced together, a larger gauge of wire is used to connect them to the grounding point. |
| 05:46 | Ignition coils are a component that needs special attention paid when we're designing our power grounding system. |
| 05:51 | As a relatively large current passes through them and they can be a strong source of electrical noise. |
| 05:56 | This is particularly important when we're wiring a direct fire ignition system that has a single coil per cylinder. |
| 06:02 | When the coil fires, the secondary winding current travels to the spark plug either via an HT lead or a direct connection for a coil on plug setup, and then jumps the gap from the centre electrode to the ground strap of the spark plug and heads out into the engine block. |
| 06:17 | From here it needs to make its way back to the coil secondary winding, as the current does flow in a closed loop. |
| 06:23 | This is why the majority of direct fire internally ignited ignition coils will have a pin that connects to one side of the secondary winding. |
| 06:32 | This pin is connected directly to the engine block, as this allows the seconday winding current a short path back to the coil, limiting the loop area this current creates and therefore also limiting the magnetic interference that it might generate. |
| 06:46 | Although the current that actually passes through the coil secondary winding circuit and spark plug gap is fairly small the speed at which this current starts and stops flowing can radiate substantial noise. |
| 06:57 | Looking at our FD3S example, the ECU low side switches the injectors, control solenoids, O2 sensor heater, oil metering pump motor and the fuel pump relay. |
| 07:07 | So we do not need to consider these elements in our grounding design. |
| 07:10 | We'll ground the ECU to the engine block. |
| 07:13 | The extra required current capacity is catered for by the ECU having four power ground pins, as opposed to its two power supply pins. |
| 07:21 | All of these power ground pins are run separately to our star earthing point and not spliced together until they reach the eyelet connector at the engine block. |
| 07:30 | The IGN-1A coils we are using have a built in ignition module. |
| 07:34 | So they pass their own current to ground. |
| 07:36 | We will also ground these directly to the engine block with 18 AWG wire, which matches the power supply wiring to each coil. |
| 07:44 | The secondary winding pins of the ignition coil will also be grounded to the star earth point on the engine block, but we will splice these wires together at the transition point where the individual ignition coil harness sections branch out, and run them back to the earth point via a single 22 AWG wire. |
| 08:01 | We can do this because the current flowing in this circuit is very low and intermittent. |
| 08:06 | We will not have a situation where two spark events are occurring at the same time. |
| 08:11 | The fuel pump is high side switched by its relay and we're going to ground the fuel pump to the chassis of the vehicle near the fuel tank, as this is more practical with the pump being at the opposite end of the car. |
| 08:21 | As mentioned, we're going to use 12 AWG wire for this to cater for future modifications. |
